Air conditioner

ABSTRACT

In an air conditioner having a double-pipe heat exchanger in a refrigerating cycle, the degree of opening of a bypass expansion valve is controlled easily without liquid return to a compressor and without considering the state of a low-pressure two-phase refrigerant in the double-pipe heat exchanger. In the air conditioner including a refrigerating cycle  1 A in which a heating expansion valve  51  is provided between the outdoor heat exchanger  30  and the double-pipe heat exchanger  60 ; a cooling expansion valve  52  is provided between the indoor heat exchanger  40  and the double-pipe heat exchanger  60 ; and in the double-pipe heat exchanger  60 , a high-pressure liquid refrigerant flowing in a liquid-side refrigerant pipe  50  is heat-exchanged with a low-pressure two-phase refrigerant that is formed by decompressing some of the high-pressure liquid refrigerant by a bypass expansion valve  62 , a low-pressure refrigerant outflow portion  60   a  of the double-pipe heat exchanger  60  is connected to a refrigerant pipe portion  50   a  between the outdoor heat exchanger  30  and the heating expansion valve  51  via a first check valve  71 , and is connected to a refrigerant pipe portion  50   b  between the indoor heat exchanger  40  and the cooling expansion valve  52  via a second check valve  72.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on, and claims priority from, JapaneseApplication Serial Number JP2009-293650, filed Dec. 25, 2009, thedisclosure of which is hereby incorporated by reference herein in itsentirety.

TECHNICAL FIELD

The present invention relates to an air conditioner having a reversiblerefrigerating cycle. More particularly, it relates to an air conditionerhaving a double-pipe heat exchanger in a liquid-side refrigerant pipeconnecting an outdoor heat exchanger and an indoor heat exchanger toeach other.

BACKGROUND ART

As one of refrigerating cycles applied to an air conditioner, arefrigerating cycle having a double-pipe heat exchanger to increase thedegree of supercooling has been known. In the refrigerating cycle ofthis type, some of a high-pressure liquid refrigerant condensed by acondenser is split and decompressed, and is heat-exchanged with amainstream high-pressure liquid refrigerant. One example thereof isexplained with reference to FIG. 3.

A refrigerating cycle 1B of this conventional example includes, as abasic configuration, a compressor 10, a four-way valve 20, an outdoorheat exchanger 30, and an indoor heat exchanger 40, and the dischargeside of the compressor 10 is connected to either one of the outdoor heatexchanger 30 and the indoor heat exchanger 40 via the four-way valve 20.

That is, at the time of cooling operation, the discharge side of thecompressor 10 is connected to the outdoor heat exchanger 30, the outdoorheat exchanger 30 functions as a condenser, and the indoor heatexchanger 40 functions as an evaporator. At the time of heatingoperation, contrarily, the discharge side of the compressor 10 isconnected to the indoor heat exchanger 40, the indoor heat exchanger 40functions as a condenser, and the outdoor heat exchanger 30 functions asan evaporator.

In both the cases, in a pipe leading from the four-way valve 20 to theoutdoor heat exchanger 30 and the indoor heat exchanger 40, a gasrefrigerant is caused to flow, and in a refrigerant pipe 11 leading fromthe four-way valve 20 to an accumulator 12 as well, the gas refrigerantis caused to flow. Therefore, these pipes are called gas-siderefrigerant pipes.

In contrast, in a refrigerant pipe connecting the outdoor heat exchanger30 and the indoor heat exchanger 40 to each other, a condensed liquidrefrigerant is mainly caused to flow. Therefore, the refrigerant pipeconnecting the outdoor heat exchanger 30 and the indoor heat exchanger40 to each other is usually called a liquid-side refrigerant pipe 50.

The liquid-side refrigerant pipe 50 is provided with a double-pipe heatexchanger 60. Also, between the double-pipe heat exchanger 60 and theoutdoor heat exchanger 30, a heating expansion valve 51 is provided, andbetween the double-pipe heat exchanger 60 and the indoor heat exchanger40, a cooling expansion valve 52 is provided.

The double-pipe heat exchanger 60 consists, for example, of an innerpipe and an outer pipe arranged coaxially, and a high-pressure liquidrefrigerant is caused to flow in the inner pipe. To the outer pipe, abypass pipe 61 branched from the liquid-side refrigerant pipe 50 isconnected, and the bypass pipe 61 is provided with a bypass expansionvalve 62.

A two-way valve 53 and a three-way valve 54 provided on both sides ofthe indoor heat exchanger 40 are connection pipes for connecting theindoor heat exchanger 40 to the refrigerating cycle when the airconditioner is installed.

At the time of cooling operation, the heating expansion valve 51 isfully opened, and the cooling expansion valve 52 is throttled to apredetermined degree of opening, so that the refrigerant flows asindicated by the solid-line arrow marks in FIG. 3. At the time ofheating operation, the cooling expansion valve 52 is fully opened, andthe heating expansion valve 51 is throttled to a predetermined degree ofopening, so that the refrigerant flows as indicated by the broken-linearrow marks in FIG. 3.

In both the operations, in the inner pipe of the double-pipe heatexchanger 60, the high-pressure liquid refrigerant (mainstream)condensed by the outdoor heat exchanger 30 or the indoor heat exchanger40 is caused to flow. In the outer pipe thereof, a low-pressuretwo-phase refrigerant that is split from the mainstream high-pressureliquid refrigerant and decompressed by the bypass expansion valve 62 iscaused to flow. The low-pressure two-phase refrigerant is heat-exchangedwith the mainstream high-pressure liquid refrigerant and is evaporated,and the mainstream high-pressure liquid refrigerant is cooled. In thiscase, the degree of opening of the bypass expansion valve 62 iscontrolled so that the degree of supercooling of the high-pressureliquid refrigerant becomes a target degree of supercooling.

As described above, the low-pressure two-phase refrigerant is evaporatedby the heat exchange with the high-pressure liquid refrigerant, and isreturned to a suction pipe 11 of the compressor 10 as a low-pressure gasrefrigerant (for example, refer to Japanese Patent ApplicationPublication No. 2006-23073).

Unfortunately, in the above-described conventional example, since thegas refrigerant evaporated by the heat exchange with the high-pressureliquid refrigerant in the double-pipe heat exchanger 60 is returned tothe suction pipe 11 side of the compressor 10, there arise problemsdescribed below in controlling the bypass expansion valve 62 so that thedegree of supercooling of high-pressure liquid refrigerant becomes thetarget degree of supercooling.

With reference to the Mollier chart of FIG. 4, the case of coolingoperation is explained. In FIG. 4, the solid line indicates themainstream of the high-pressure liquid refrigerant flowing in theliquid-side refrigerant pipe 50, and the dash-and-dot line indicates abypass stream flowing in the bypass pipe 61.

In particular, in the case where a pipe for connecting an outdoor unitand an indoor unit to each other must be lengthened on account of thecircumstances of the place at which the air conditioner is installed, inorder to optimize the state in which the refrigerant reaches the indoorheat exchanger 40 (to demonstrate the capacity of indoor unit to amaximum), supercooling as shown in FIG. 4A is needed.

FIG. 4A shows a refrigerating cycle in which the refrigerant circulatesin the optimum state. Even if the refrigerant reaches the indoor heatexchanger 40 in the optimum state with the degree of supercooling beingA and the mainstream and the bypass stream are mixed with each other, astate of gas phase is established.

That is, the low-pressure two-phase refrigerant that is split from themainstream and decompressed by the bypass expansion valve 62 evaporatesin the double-pipe heat exchanger 60, and becomes in an overheated stateof (c1). The mainstream evaporates in the indoor heat exchanger 40 andreturns to the compressor 10 in the state of (a1), and on the suctionside of the compressor 10, (a1) and (c1) are mixed with each other, andthe state of (b1) is formed.

On the other hand, as shown in FIG. 4B, in the case where the bypassamount to the double-pipe heat exchanger 60 is increased to change thedegree of supercooling deep from A to A¢ (in the left direction in FIG.4B) because the refrigerant reaching the indoor unit is not optimal, themainstream is heat-exchanged sufficiently by the indoor heat exchanger40, and a gas phase (a2) is formed.

However, if all of the bypass refrigerant becomes impossible toevaporate, the refrigerant is returned in the two-phase state of (c2) inwhich the degree of overheating is zero. Therefore, the refrigerantmixed on the suction side of the compressor 10 becomes in the two-phasestate (b2) containing the liquid refrigerant, and liquid back occurs.

Accordingly, in order to make (b2) in a gas-phase state to avoid liquidback, the degree of supercooling must be made shallow (from A¢ to A, inthe right direction in FIG. 4B). In this case, the refrigerant does notreach the indoor heat exchanger 40 in the optimum state, and theperformance (COP) deteriorates.

Thus, in the above-described example, since there is a fear of liquidreturn to the compressor 10, the temperature of the bypass stream at theoutlet of the double-pipe heat exchanger 60 is monitored to suppress theflow rate of bypass stream. As a result, there occurs the case where thetarget degree of supercooling is not reached. Also, the circulationamount of refrigerant in the evaporator (for example, the indoor heatexchanger 40) is only the amount of the mainstream, so that the heatexchange amount sometimes comes short.

As one method for making the bypass stream a gas refrigerant byevaporating all of the bypass stream, a method is available in which thedouble-pipe heat exchanger 60 is increased in size. However, this methodis unfavorable because the piping system becomes large in size.

Accordingly, an object of the present invention is to provide an airconditioner having a double-pipe heat exchanger in a refrigeratingcycle, wherein the degree of opening of a bypass expansion valve can becontrolled easily without liquid return to a compressor and withoutconsidering the state of a low-pressure two-phase refrigerant in thedouble-pipe heat exchanger.

SUMMARY OF THE INVENTION

To achieve the above object, the present invention provides an airconditioner including a refrigerating cycle in which a double-pipe heatexchanger is provided in a liquid-side refrigerant pipe between anoutdoor heat exchanger and an indoor heat exchanger which areselectively connected to the discharge side of a compressor via afour-way valve; a heating expansion valve is provided between theoutdoor heat exchanger and the double-pipe heat exchanger; a coolingexpansion valve is provided between the indoor heat exchanger and thedouble-pipe heat exchanger; and in the double-pipe heat exchanger, ahigh-pressure liquid refrigerant flowing in the liquid-side refrigerantpipe is heat-exchanged with a gas-liquid low-pressure two-phaserefrigerant which is formed by decompressing some of the high-pressureliquid refrigerant by a bypass expansion valve, wherein a low-pressurerefrigerant outflow portion of the double-pipe heat exchanger isbranched in a fork form; one branch is connected to the refrigerant pipebetween the outdoor heat exchanger and the heating expansion valve viafirst valve means; and the other branch is connected to the refrigerantpipe between the indoor heat exchanger and the cooling expansion valvevia second valve means.

In the present invention, at the time of cooling operation of therefrigerating cycle, the heating expansion valve is fully opened and thecooling expansion valve is throttled to a predetermined degree ofopening; and the low-pressure refrigerant heat-exchanged by thedouble-pipe heat exchanger is supplied to the indoor heat exchanger onthe evaporator side via the second valve means together with therefrigerant decompressed by the cooling expansion valve.

Also, at the time of heating operation of the refrigerating cycle, thecooling expansion valve is fully opened and the heating expansion valveis throttled to a predetermined degree of opening; and the low-pressurerefrigerant heat-exchanged by the double-pipe heat exchanger is suppliedto the outdoor heat exchanger on the evaporator side via the first valvemeans together with the refrigerant decompressed by the heatingexpansion valve.

In the present invention, as the first and second valve means, checkvalves which are opened with the low-pressure refrigerant outflowportion being on the high pressure side or solenoid valves which areopened and closed by an external signal may be used.

According to the present invention, since the low-pressure refrigerantgoing out of the double-pipe heat exchanger is caused to flow to theevaporator side, the refrigerant is evaporated by the evaporator and isreturned to the compressor even if not being evaporated completely bythe double-pipe heat exchanger. Therefore, liquid return to thecompressor can be eliminated.

Also, in controlling the bypass expansion valve, the state of thelow-pressure refrigerant in the double-pipe heat exchanger need not beconsidered, and the control has only to be carried out so that thedegree of supercooling of the high-pressure liquid refrigerant becomes atarget degree of supercooling. Therefore, the bypass expansion valve canbe controlled easily.

Also, since a large amount of low-pressure refrigerant can be caused toflow in the double-pipe heat exchanger, the degree of supercooling ofthe high-pressure liquid refrigerant can be made high, so that theimprovement in performance of the refrigerating cycle can be expectedaccordingly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a refrigerant circuit diagram showing an embodiment of arefrigerating cycle applied to an air conditioner of the presentinvention;

FIG. 2 is a Mollier chart of the refrigerating cycle shown in FIG. 1;

FIG. 3 is a refrigerant circuit diagram showing a conventionalrefrigerating cycle;

FIG. 4A is a Mollier chart in the case where the degree of overheatingis established in a double-pipe heat exchanger of the conventionalrefrigerating cycle shown in FIG. 3; and

FIG. 4B is a Mollier chart in the case where the degree of overheatingis not established in a double-pipe heat exchanger of the conventionalrefrigerating cycle shown in FIG. 3.

DETAILED DESCRIPTION

An embodiment of the present invention will now be described withreference to FIGS. 1 and 2. The present invention is not limited to thisembodiment. In the explanation of this embodiment, the same referencenumerals are applied to elements that are essentially the same as theelements in the conventional example explained with reference to FIG. 3.

As shown in FIG. 1, a refrigerating cycle 1A in accordance with thisembodiment includes, as a basic configuration, a compressor 10, afour-way valve 20, an outdoor heat exchanger 30, and an indoor heatexchanger 40. The compressor 10 may be either a rotary compressor or ascroll compressor.

The discharge side of the compressor 10 is connected to either one ofthe outdoor heat exchanger 30 and the indoor heat exchanger 40 via thefour-way valve 20, and in a liquid-side refrigerant pipe 50 thatconnects the outdoor heat exchanger 30 and the indoor heat exchanger 40to each other, a double-pipe heat exchanger 60 is interposed.

Also, between the outdoor heat exchanger 30 and the double-pipe heatexchanger 60, a heating expansion valve 51 is provided, and between theindoor heat exchanger 40 and the double-pipe heat exchanger 60, acooling expansion valve 52 is provided.

The double-pipe heat exchanger 60 consists, for example, of an innerpipe and an outer pipe arranged coaxially, and a high-pressure liquidrefrigerant condensed by the outdoor heat exchanger 30 or the indoorheat exchanger 40 is caused to flow in the inner pipe. Thishigh-pressure liquid refrigerant caused to flow in the inner pipe is amainstream.

To the outer pipe of the double-pipe heat exchanger 60, a bypass pipe 61branched from the liquid-side refrigerant pipe 50 is connected, and thebypass pipe 61 is provided with a bypass expansion valve 62. Some of thehigh-pressure liquid refrigerant split from the bypass pipe 61 isdecompressed, and flows in the outer pipe as a low-pressure two-phaserefrigerant. The high-pressure liquid refrigerant may be caused to flowon the outer pipe side, and the low-pressure two-phase refrigerant maybe caused to flow on the inner pipe side.

According to the present invention, a low-pressure refrigerant outflowportion 60 a of the double-pipe heat exchanger 60 is connected to arefrigerant pipe portion 50 a between the outdoor heat exchanger 30 andthe heating expansion valve 51 via a first check valve 71, and is alsoconnected to a refrigerant pipe portion 50 b between the indoor heatexchanger 40 and the cooling expansion valve 52 via a second check valve72.

In both the check valves 71 and 72, the forward direction of flow is adirection directed from the low-pressure refrigerant outflow portion 60a to the refrigerant pipe portions 50 a and 50 b. In place of the checkvalve, a solenoid valve that is opened and closed by an external signalmay be used.

At the time of cooling operation, the four-way valve 20 is changed overto the state indicated by the solid line in FIG. 1. In this state, theheating expansion valve 51 is fully opened, the cooling expansion valve52 is throttled to a predetermined degree of opening, and therefrigerant circulates as indicated by the solid-line arrow marks inFIG. 1.

That is, a high-pressure gas refrigerant discharged from the compressor10 reaches the outdoor heat exchanger 30 through the four-way valve 20,being condensed into the high-pressure liquid refrigerant by the outdoorheat exchanger 30, and is further cooled by the double-pipe heatexchanger 60.

The high-pressure liquid refrigerant from which the degree ofsupercooling is removed by the double-pipe heat exchanger 60 is split ina portion of the bypass pipe 61. One stream (the mainstream) is sent tothe cooling expansion valve 52, and the other stream (a bypass stream)is sent to the bypass expansion valve 62.

The bypass stream is decompressed into the gas-liquid low-pressuretwo-phase refrigerant by the bypass expansion valve 62, and isheat-exchanged with the high-pressure liquid refrigerant by thedouble-pipe heat exchanger 60 and is evaporated.

At the time of cooling operation, the refrigerant in the refrigerantpipe portion 50 a on the outdoor heat exchanger 30 side has a pressurehigher than the pressure of refrigerant at the low-pressure refrigerantoutflow portion 60 a, and the refrigerant in the refrigerant pipeportion 50 b on the indoor heat exchanger 40 side has a pressure lowerthan the pressure of refrigerant at the low-pressure refrigerant outflowportion 60 a.

Therefore, the evaporated gas refrigerant reaches the refrigerant pipeportion 50 b via the second check valve 72, joining with themainstream-side refrigerant decompressed by the cooling expansion valve52, and is sent to the indoor heat exchanger 40 on the evaporator side.In the indoor heat exchanger 40, the refrigerant is heat-exchanged withindoor air and is evaporated, and the gas refrigerant is returned to thecompressor 10 through a suction pipe 11 and an accumulator 12.

At the time of cooling operation, the flow rate of refrigerant in therefrigerating cycle is regulated by the cooling expansion valve 52, andthe degree of opening of the bypass expansion valve 62 is controlled sothat the degree of supercooling of the high-pressure liquid refrigerantbecomes a target degree of supercooling.

At the time of heating operation, the four-way valve 20 is changed overto the state indicated by the broken line in FIG. 1. In this state, thecooling expansion valve 52 is fully opened, the heating expansion valve51 is throttled to a predetermined degree of opening, and therefrigerant circulates as indicated by the broken-line arrow marks inFIG. 1.

That is, the high-pressure gas refrigerant discharged from thecompressor 10 reaches the indoor heat exchanger 40 through the four-wayvalve 20, and is condensed into the high-pressure liquid refrigerant bythe indoor heat exchanger 40.

Thereafter, the high-pressure liquid refrigerant is split in the portionof the bypass pipe 61 in front of the double-pipe heat exchanger 60. Onestream (the mainstream) flows in the inner pipe of the double-pipe heatexchanger 60 and reaches the heating expansion valve 51, and the otherstream (the bypass stream) is sent to the bypass expansion valve 62.

The bypass stream is decompressed into the gas-liquid low-pressuretwo-phase refrigerant by the bypass expansion valve 62, and isheat-exchanged with the high-pressure liquid refrigerant on themainstream side by the double-pipe heat exchanger 60 and is evaporated.

At the time of heating operation, the refrigerant in the refrigerantpipe portion 50 b on the indoor heat exchanger 40 side has a pressurehigher than the pressure of refrigerant at the low-pressure refrigerantoutflow portion 60 a, and the refrigerant in the refrigerant pipeportion 50 a on the outdoor heat exchanger 30 side has a pressure lowerthan the pressure of refrigerant at the low-pressure refrigerant outflowportion 60 a.

Therefore, the evaporated gas refrigerant reaches the refrigerant pipeportion 50 a via the first check valve 71, joining with themainstream-side refrigerant decompressed by the heating expansion valve51, and is sent to the outdoor heat exchanger 30 on the evaporator side.In the outdoor heat exchanger 30, the refrigerant is heat-exchanged withthe outside air and is evaporated, and the gas refrigerant is returnedto the compressor 10 through the suction pipe 11 and the accumulator 12.

At the time of heating operation as well, the flow rate of refrigerantin the refrigerating cycle is regulated by the heating expansion valve51, and the degree of opening of the bypass expansion valve 62 iscontrolled so that the degree of supercooling of the high-pressureliquid refrigerant becomes the target degree of supercooling.

As described above, according to the present invention, at both of thecooling operation time and the heating operation time, since thelow-pressure refrigerant going out of the double-pipe heat exchanger 60is caused to flow from the downstream side of the cooling expansionvalve 52 or the heating expansion valve 51 toward the evaporator, thegas-liquid low-pressure two-phase refrigerant need not be evaporatedcompletely in the double-pipe heat exchanger 60. Therefore, a largeamount of low-pressure two-phase refrigerant can be caused to flow inthe double-pipe heat exchanger 60 by increasing the target degree ofsupercooling of the high-pressure liquid refrigerant.

With reference to the Mollier chart of FIG. 2, the refrigerating cycleof the present invention (the case of cooling operation) is explained.In FIG. 2, the solid line indicates the mainstream of the high-pressureliquid refrigerant flowing in the liquid-side refrigerant pipe 50, andthe dash-and-dot line indicates the bypass stream flowing in the bypasspipe 61.

At point D, the refrigerant is sucked into the compressor 10, and thecompressed refrigerant becomes at high temperature and pressure (pointx) and is condensed by the outdoor heat exchanger 30 (point a). Therefrigerant is heat-exchanged with the later-described bypass stream(g-f) by the double-pipe heat exchanger 60 and becomes in a supercooledstate (point b), and is decompressed by the cooling expansion valve 52(point d).

On the other hand, in the bypass circuit, some of the mainstreamheat-exchanged by the double-pipe heat exchanger 60 is split in thebypass pipe 61, and is decompressed by the bypass expansion valve 62(point g). Thereafter, the refrigerant is heat-exchanged with themainstream (a-b) (point f). The mainstream and the bypass stream arejoined with each other (point e) and flow into the indoor heat exchanger30. The refrigerant is evaporated by the indoor heat exchanger 30, andis sucked into the compressor 10 (point D).

In the present invention, as described above, the bypass stream used inthe double-pipe heat exchanger 60 does not return directly to thesuction side of the compressor 10, and is heat-exchanged by the indoorheat exchanger 30, so that no wasteful refrigerant is generated.Therefore, the performance (COP) is improved. Also, since there is nofear of liquid back, supercooling can be performed until the refrigerantcan be supplied to the indoor heat exchanger 30 in the optimum state.

Also, conventionally, both of the electronic expansion valve fordouble-pipe heat exchanger and the electronic expansion valves of thewhole of refrigerating cycle have been needed to be controlled exactly.According to the present invention, however, since liquid back does notoccur, the control program for these electronic expansion valves can besimplified significantly.

1. An air conditioner including a refrigerating cycle in which adouble-pipe heat exchanger is provided in a liquid-side refrigerant pipebetween an outdoor heat exchanger and an indoor heat exchanger which areselectively connected to the discharge side of a compressor via afour-way valve; a heating expansion valve is provided between theoutdoor heat exchanger and the double-pipe heat exchanger; a coolingexpansion valve is provided between the indoor heat exchanger and thedouble-pipe heat exchanger; and in the double-pipe heat exchanger, ahigh-pressure liquid refrigerant flowing in the liquid-side refrigerantpipe is heat-exchanged with a gas-liquid low-pressure two-phaserefrigerant which is formed by decompressing some of the high-pressureliquid refrigerant by a bypass expansion valve, wherein a low-pressurerefrigerant outflow portion of the double-pipe heat exchanger isbranched in a fork form; one branch is connected to the refrigerant pipebetween the outdoor heat exchanger and the heating expansion valve viafirst valve means; and the other branch is connected to the refrigerantpipe between the indoor heat exchanger and the cooling expansion valvevia second valve means.
 2. The air conditioner according to claim 1,wherein at the time of cooling operation of the refrigerating cycle, theheating expansion valve is fully opened and the cooling expansion valveis throttled to a predetermined degree of opening; and the low-pressurerefrigerant heat-exchanged by the double-pipe heat exchanger is suppliedto the indoor heat exchanger on the evaporator side via the second valvemeans together with the refrigerant decompressed by the coolingexpansion valve.
 3. The air conditioner according to claim 1, wherein atthe time of heating operation of the refrigerating cycle, the coolingexpansion valve is fully opened and the heating expansion valve isthrottled to a predetermined degree of opening; and the low-pressurerefrigerant heat-exchanged by the double-pipe heat exchanger is suppliedto the outdoor heat exchanger on the evaporator side via the first valvemeans together with the refrigerant decompressed by the heatingexpansion valve.
 4. The air conditioner according to claim 1, whereinthe first and second valve means consist of check valves which areopened with the low-pressure refrigerant outflow portion being on thehigh pressure side.
 5. The air conditioner according to claim 1, whereinthe first and second valve means consist of solenoid valves which areopened and closed by an external signal.